A system's galactic neighbors will throw orbits into chaos.

In the last two years, astronomers discovered several exoplanets in binary systems: two stars locked in mutual orbit. These systems come in several types, with the planet orbiting one or both stars. About half the binary systems involve stars that are very far apart: 1000 times the Earth-Sun separation or more. Naively, we might think that those systems are more stable, since the companion star is so far away. However, a new study shows that may not be the case.

Nathan A. Kaib, Sean N. Raymond, and Martin Duncan ran extensive computer simulations to model exoplanets residing in wide binary systems. They found that perturbations from other stars outside the binary system had a profound effect on the shape of the system's orbits. In some cases, planets were ejected from the system entirely or ended up in larger or highly eccentric (elongated) orbits. Based on these results, the researchers argued that some of the observed exoplanet systems with eccentric orbits may actually reside in wide binary systems where we haven't yet detected the companion stars.

The left side shows the orbit of a small companion star as it's influenced by the passage of neighboring stars. The right shows the chaos it would inflict on the orbits of the outer planets of our solar system. (Video courtesy of Nathan Kaib)

A significant fraction of stars in the Milky Way are in binary systems. Some, like the Alpha Centauri system, are tight binaries: the two stars comprising Alpha Centauri are about 18 astronomical units (AU) apart. (1 AU is Earth's average distance to the Sun. For comparison, Neptune's average distance is about 30 AU.) However, some are known as wide binaries, with separations greater than 1000 AU.

In wide binaries, the mutual gravitational attraction between the stars is relatively weak. This allows other, neighboring stars to have a significant influence. As the other stars of the Milky Way gradually shift in their slow orbits around the galactic center, their gravitational influence on the binary fluctuates in time. The effect is to change the shape and size of the wide binary system, altering the mutual orbit over periods of billions of years.

While the process isn't exactly rapid, the new study revealed it can have a profound impact if there are planets orbiting one of the stars in the binary. Alteration of one star's trajectory increased both the size and eccentricity of the planets' orbits. Over a simulation period of about 10 billion years, 30 to 60 percent of systems designed to resemble our Solar System lost one or more planets, leaving the remaining planets in vastly different configurations.

According to the standard models of planetary formation, planets form in regular, circular orbits—ones with nearly zero eccentricity, in other words. However, observations have found many exoplanets are in highly eccentric orbits, and many are much closer to their host star than the naive planet-formation scenario would suggest.

The authors of the study presented their simulations as a possible resolution to some of these problems. If the exoplanets in eccentric orbits actually are in wide binaries—in which the companion star is undetected—then their strange orbits were caused by gravitational perturbations from the natural cycles of the Milky Way. This idea is also in agreement with an earlier paper, which posits that retrograde orbits of some exoplanets—planets orbiting opposite to their star's rotation—could be explained if there once was a binary companion star that is now absent.

These results also suggest that planetary orbits in wide binaries may be less stable over billions of years than they would be in tight binaries. If the two stars are closer together, the authors argued, they are less subject to disturbance from the ebb and flow of external gravitational influence. Of course, any planetary system with three or more interacting objects exhibits complex behavior, given long enough time; even the Solar System cannot be proven to be stable forever. (The fact that it has been relatively stable for 4.5 billion years is no guarantee that state will continue in perpetuity.)

The wide-binary model is certainly testable with further observations. Faint companion stars farther than 1000 AU would be hard to detect or challenging to prove they are gravitationally bound to the exoplanet system. However, if they could be identified for at least some eccentric exoplanet orbits, that would lend a lot of support to the proposed model.

IIRC, the structure of our solar system is not quite consistent with the current theories of solar system formation... unless you include a transit of a neighboring star nearby sometime early in solar system formation. Without that nearby transit, when they model solar system formation the planets all end up in the "wrong" places.

Specifically I was wondering if a binary star system could be considered similar to the Sun-Jupiter pair. Since there is a Trojan point (2 actually, L4 and L5), could a planet form/be captured at this location, similar to the stable(?) Trojan asteroids of Jupiter? As I understand it, there must be a minimum mass ratio of primary objects (25:1 or there abouts) Could two stars that are closer in mass form a stable L4/L5 Trojan point?

This idea is also in agreement with an earlier paper, which posits that retrograde orbits of some exoplanets—planets orbiting opposite to their star's rotation—could be explained if there once was a binary companion star that is now absent.

That other star can't just go for a walk, the only ways I can see it becoming absent is if it were to explode (then we'd have some not-so subtle remnants) or if there was a third body (of stellar mass) involved. Further, as near as I can tell, your normal three-body ejection requires that they all be gravitationally bound, so there will still be 2 stars left. As for a near miss, I'm not clear that it would have any great effect on a wide binary unless it was extremely close to one of the stars, which just ain't that likely near as I can tell.

When 2 galaxies collide, I wonder if it's likely to result in any direct stellar collisions? My understanding is that this is not expected to occur.

Specifically I was wondering if a binary star system could be considered similar to the Sun-Jupiter pair. Since there is a Trojan point (2 actually, L4 and L5), could a planet form/be captured at this location, similar to the stable(?) Trojan asteroids of Jupiter? As I understand it, there must be a minimum mass ratio of primary objects (25:1 or there abouts) Could two stars that are closer in mass form a stable L4/L5 Trojan point?

You could place a planet in the L4/L5 Lagrange but this planet would be completely uninhabitable due to extreme distance from either star in the sort of binary systems that this article is adressing. Also, I believe that such planets are completely undetectable with current technology and such a planet would have the same period of rotation about the center of gravity as both the stars as I mentioned above.

As far as the mass ratio for Trojans goes it is mostly a matter of how much smaller you decide you want the Trojan to be before you can approximate it as having no effect on the system. You might have very loose requirements if you only want the system to be stable for a hundred million years or very tight requirements if you want it to stay stable for many billions of years.

IIRC, the structure of our solar system is not quite consistent with the current theories of solar system formation... unless you include a transit of a neighboring star nearby sometime early in solar system formation. Without that nearby transit, when they model solar system formation the planets all end up in the "wrong" places.

??? References needed.

The current Nice model is well known to be based on the current theories of solar system formation while remaining very predictive on the features of system. Recent additions with at least one ejected Neptune admits a larger disk and predicts the system and many of its moons and all its debris disks out to the inner edge of the Kuiper belt. [ http://en.wikipedia.org/wiki/Nice_model ]

The rest from the distance of the outer edge of the Kuiper belt to the size of the Oort cloud an such features as Eris's orbit, and the metallicity and early radioisotopes* of the Sun & Earth, is predicted by a recent 2 stage model for the molecular cloud that birthed the Sun. I commented on it here not long ago, so I'll leave googling that as ref for the time being.

In fact, we may very well have a largely self-consistent understanding of our system now. And it excludes external interactions, which would be rare anyway.

* Unless a even more recent precision measurement and subsequent change of the isotope ratios will stand. But I don't think it is accepted as of yet.

"It is also possible for widely separated binaries to lose gravitational contact with each other during their lifetime, as a result of external perturbations. The components will then move on to evolve as single stars. A close encounter between two binary systems can also result in the gravitational disruption of both systems, with some of the stars being ejected at high velocities, leading to runaway stars.[57][58]"

"It is also possible for widely separated binaries to lose gravitational contact with each other during their lifetime, as a result of external perturbations. The components will then move on to evolve as single stars. A close encounter between two binary systems can also result in the gravitational disruption of both systems, with some of the stars being ejected at high velocities, leading to runaway stars.[57][58]"

So we have a 3-body (or more) interaction, same as I said. A binary star system by itself cannot just lose a star.

Also I'm not clear if these "widely separated" binaries are separated anywhere near the 1000AU mentioned in the article, is this possibility even relevant here?

When 2 galaxies collide, I wonder if it's likely to result in any direct stellar collisions? My understanding is that this is not expected to occur.

Direct stellar collisions would be extremely rare, but there would almost certainly be "some". When the Milky Way and Andromeda finally impact each other, even if the odds of collisions are a billion-to-one against, you'd still expect a few hundred collisions.

But the point astrophysicists would make is that such extremely rare stellar collisions would not affect how a galactic merger event would proceed. There would be way too few stellar collisions to affect the evolution of the merging galaxies.

When 2 galaxies collide, I wonder if it's likely to result in any direct stellar collisions? My understanding is that this is not expected to occur.

Not likely, same as for collisions whan stars migrate in the current galactic disk.

Not really the same thing as stars migrating in the current galactic disk. In a galactic disk, all stars are orbiting in the same direction, usually with roughly the same orbits. Stars still jostle around with each other, but its really nearby orbits perturbing each other. But when two galaxies collide, you have billions of stars trying to cross paths with another billions of stars.

Here's an analogy: Take a handful of marbles and roll them across a room. Some of them will impact each other, but most will not. The ones that do impact each other will generally only glance off each other (this would be the equivalent of stellar near-misses in similar galactic orbits). Now have a buddy on the other side of the room roll a handful of marbles towards you at the same time you roll your handful towards him. There will still not be a ton of impacts, but there will be more and you'll get a few dramatic head-on collisions. (In this thought experiment, imagine doing it a bunch of times. The rate of head-on collisions will be low. Now remember that a galactic collision plays out with billions of stars. A really low rate of collisions will still result in "some" collisions, won't it?)

EDIT: Again, my point is that the odds of any particular star being involved in a collision is tiny, tiny, tiny. But the odds of there being at least a few stellar collisions during a galactic merger approaches one.

IIRC, the structure of our solar system is not quite consistent with the current theories of solar system formation... unless you include a transit of a neighboring star nearby sometime early in solar system formation. Without that nearby transit, when they model solar system formation the planets all end up in the "wrong" places.

??? References needed.

The current Nice model is well known to be based on the current theories of solar system formation while remaining very predictive on the features of system. Recent additions with at least one ejected Neptune admits a larger disk and predicts the system and many of its moons and all its debris disks out to the inner edge of the Kuiper belt. [ http://en.wikipedia.org/wiki/Nice_model ]

The rest from the distance of the outer edge of the Kuiper belt to the size of the Oort cloud an such features as Eris's orbit, and the metallicity and early radioisotopes* of the Sun & Earth, is predicted by a recent 2 stage model for the molecular cloud that birthed the Sun. I commented on it here not long ago, so I'll leave googling that as ref for the time being.

In fact, we may very well have a largely self-consistent understanding of our system now. And it excludes external interactions, which would be rare anyway.

* Unless a even more recent precision measurement and subsequent change of the isotope ratios will stand. But I don't think it is accepted as of yet.

Could be out of date. It was a Scientific American article some years ago. I generally try to keep up on this stuff, but us amateurs can't keep up with everything. ;-)

When 2 galaxies collide, I wonder if it's likely to result in any direct stellar collisions? My understanding is that this is not expected to occur.

Not likely, same as for collisions whan stars migrate in the current galactic disk.

Not really the same thing as stars migrating in the current galactic disk. In a galactic disk, all stars are orbiting in the same direction, usually with roughly the same orbits. Stars still jostle around with each other, but its really nearby orbits perturbing each other. But when two galaxies collide, you have billions of stars trying to cross paths with another billions of stars.

It might sound like a lot, but also keep in mind there's a lot of distance as well. Consider a dense collection of stars like you'd find in the nebula NGC 3603, where 10,000 stars are packed within 3 lightyears of each other. It might sound crazy, but none of these stars are in danger of colliding. Why? Because even though it sounds small in the scale of lightyears, it's actually friggin huge! That's a TON of space! Hell, you could probably throw in another ten thousand and not have too much to worry about.

Then if you extrapolate this to the rest of the galaxy, well, it's an exception! Most of the galaxy is waaaay emptier than that. Sure, it has billions of stars and, sure, Andromeda has even more billions of stars, but their size MORE than makes up for that number. Like, way way more than makes up for it. Absurdly more. To the point where we have to introduce Dark Matter just to explain it.

This isn't to say collisions are impossible, of course, but at worst it's going to make up >1% of what's going on in our future supergalaxy